106 METALLURGY OF IRON AND STEEL. even after the addition of large quantities of iron oxide, but with much less precision than in the open-hearth, furnace. In America, little attention has been paid in the past to the composition of the slag, as the proportion of manganese in the iron has usually been below 0.50 per cent, and the slag was thick and viscous. Within recent years the increased use of Mesabi ores has given a pig-iron carrying often 0.60 per cent, and sometimes over 1.00 per cent, of manganese. Such an iron causes much slopping during the blow, and gives a thin slag that makes it more difficult to properly recarburize the metal. Table VI-D gives the composition of slags from eight different Bessemer plants in America. Sample I was made from irons containing from 2 to 3 per cent, in silicon, while K was from irons running over 1 per cent, in manganese. TABLE VI-D. Composition of American Bessemer Slags. Si02 FeO MnO A 55.5 12.7 26.9 B 52.8 18 1 24.6 C 64.9 1 .7 9.6 D 65.8 18.5 11.7 E 62.0 16.9 12.1 F 59.7 19.3 12.0 G 62.2 20.3 13.7 H 55.5 23.0 18.0 I 69.5 15.3 9.4 K 47.0 10.1 35.5 The composition of the slag is sometimes changed by blowing with the vessel partly tipped over. This brings some of the tuyeres above the metal, so that the blast rushes over the surface, oxidizing considerable iron, and burning part of the CO to C02, so that there is a greater calorific development, and this method is taken to raise the temperature of a cold charge at the expense of a greater waste of iron, and a greater wear of the lining. Cold charges may arise from too low a content of silicon, from a low initial temperature, or from a newly repaired vessel. It is unusual in rapid American practice to have difficulty from insufficient heat, for the fastest plants will average eight heats per hour from a pair of 10-ton vessels, giving an output of 50,000 tons per month. Under these conditions one per cent, of silicon in the pig-iron is sufficient for the production of the necessary heat.